Raman radiation results from inelastic scattering. When monochromatic excitation radiation is directed to the object material, low-energy modes, such as vibration and rotation of molecules cause small deviations in the wavelength of the monochromatic radiation. Each deviation, in turn, is characteristic to each molecule in the material and hence a composition of the material can be identified.
The Raman radiation is notoriously difficult to measure since its intensity with respect to the excitation radiation is very low and it arrives at the detector almost simultaneously with the excitation radiation. Additionally, the excitation radiation gives rise to fluorescent radiation which is also simultaneous with the Raman radiation and whose lifetime is in nanosecond range which is longer than that of Raman radiation.
Notch filters are usually employed to block the excitation radiation away as effectively as possible without attenuating other wavelengths excessively. The Raman radiation has also been separated from the fluorescent radiation using a gating device in front of the detector. For example, an optically controlled Kerr-gate may be placed in front of a detector. In that way, only the photons detected when the gate is open can be counted which is used to suppress both the excitation radiation and the fluorescent radiation and the Raman radiation can be detected.
Instead of Kerr-gate, an image intensifier may correspondingly be placed in the front of the detector such as a CCD camera (Charge Coupled Device). The image intensifier can be switched on and off with a frequency in a megahertz range and with a gate period of several hundreds on picoseconds.
However, there are problems related to the prior art. The duty cycle of a Kerr-gate is low, which makes the measurement unpractical. The operation of the Kerr-gate also needs high-energy optical pulses which drastically limit the energy of the optical pulses directed to the measured object from the same optical source. Correspondingly, the image intensifier has a problem due to a difficult and contradictious requirement of forming short pulses with well over 1000V. In general measurement systems which have gating are slow, complicated, difficult or impossible to adjust, expensive and large such that they can only be used in laboratory.
Hence, there is a need for a better solution to measure Raman radiation.